A USB port supplies 5W. 4 ports means 20W, which is about 1/50th of what a 1 bar electric fire would use. Hardly taxing, is it?

It may not seem taxing but at 5V you are pulling 4 amps of power for 20W. 4A can cause significant heat through electronic devices which is why you seldom see this. Certainly it can be done but it requires heat sinks and heavy duty components. However, having said that a typical USB port only supplies .5A or 2.5W so a four port device would only have 10W, exactly what you have in a iPad charger.